A Surprising CME

A collapsing solar filament crashed into the surface
of the Sun yesterday, spawning a coronal mass ejection that could
trigger aurora here on Earth.

Sept. 13, 2000 -- On Monday,
Sept. 11, the sunspot number dropped to its lowest value of the
year. The face of the Sun looked remarkably blank with only a
few tiny spots visible in telescopes. It was a remarkable sight
considering that the Sun is well into the maximum phase of its
11-year sunspot cycle. The Sun has been almost constantly peppered
with spots since "Solar Max" began earlier this year.

Yesterday, just when it seemed that a brief interlude in solar
activity was at hand, the Sun unleashed a brilliant full-halo
coronal mass ejection (CME).

Right: Coronagraphs on board the orbiting ESA/NASA Solar and Heliospheric
Observatory recorded a full-halo coronal mass ejection at
1230 UT on Sept. 12. In this image, the faint red-colored circle
shows the true size of the Sun. The solid brick-colored region
denotes an occulting disk that blocks the bright glare of the
Sun to reveal the fainter corona. [600
kb gif animation]

"This CME appeared after a solar filament collapsed
and dropped relatively cool, dense gas onto the surface of the
Sun," says David Hathaway, a solar physicist at the NASA
Marshall Space Flight Center. "You don't need sunspots to
have filaments, so these types of explosions can occur even when
the sunspot number is low."

The CME left the Sun traveling at about 1000 km/s, says Simon
Plunkett, an operations scientist with the Solar and Heliospheric
Observatory (SOHO) coronagraph team at the Naval Research Laboratory
and the Goddard Space Flight Center. "At this speed it could
arrive sometime late Wednesday or early Thursday."

Coronal mass ejections can carry up to 10 billion tons of
electrified gas. "Halo events" like this one are CMEs
aimed in the general direction of Earth. As they loom larger
and larger they appear to envelop the Sun, forming a halo around
our star.

Although CMEs may sound menacing, they
pose little danger to Earthlings. Indeed, many skywatchers greet
news of a full-halo event with enthusiasm because CMEs can trigger
memorable displays of aurora when they strike Earth's magnetosphere
(a region of space around Earth protected by our planet's magnetic
field). The last time this happened was August 12, 2000, when
the shock wave from a CME arrived at Earth just hours before
the peak of the annual Perseid meteor shower. Legions of meteor-watchers
saw a brilliant auroral display in spite of glaring light from
that night's nearly-full Moon.

Left: Wade Clark captured this image of the Big Dipper
shining through green-colored auroral sheets in the skies above
Skagit County, Washington State on August 12, 200. The moonlit
peak is Mt. Baker.

The situation this week could be much the same. Many stargazers
will be out to see the late-summer Harvest Moon, so the CME could
arrive on a moonlit night while many people are looking at the
sky.

"The incoming CME might produce an auroral display, but
there's no guarantee," cautioned Gary Heckman of the NOAA
Space Environment Center. "There was an event very similar
to this one on July 29, 1973, during Skylab. It was a very large
flare, but it produced only a minor geomagnetic disturbance.
It's hard to make a definitive forecast because this type of
eruption is so rare."

Above Left: This image of the Sun captured at the
Big Bear Solar Observatory through a red Hydrogen-alpha filter
shows many dark linear filaments on Sept 11, 2000. One of them
near the central meridian is missing in the Sept 12th image (Above
Right). During the interval between the two exposures, the
filament collapsed, spawning a powerful x-ray solar flare and
a brilliant full-halo coronal mass ejection.

The filament that collapsed yesterday was one of many on the
Sun that space weather forecasters have been monitoring. With
very few sunspots in evidence this week, collapsing filaments
are the most likely source of solar activity.

"Filaments
are formed in magnetic loops that hold relatively cool, dense
gas suspended above the surface of the Sun," explains Hathaway.
"When you look down on top of them they appear dark because
the gas inside is cool compared to the hot photosphere below.
But when we see a filament in profile against the dark sky it
looks like a giant glowing loop -- these are called prominences
and they can be spectacular."

Right: Last year extreme ultraviolet cameras on board
the orbiting Solar and Heliospheric Observatory captured this
image of a prominence above the eastern limb of the Sun. [more information]

"Filaments collapse when the magnetic field in their
vicinity becomes unstable," explained Hathaway. "This
could happen, for example, if new magnetic field lines begin
to poke through the Sun's surface beneath the filament."
The resulting explosions, which often occur well away from spotted
regions, are called Hyder flares, named for Charles Hyder, who
published studies of such events in 1967.

NASA astronauts are currently in orbit on a mission to prepare
the International Space Station (ISS) for habitation. Are they
in any danger from the approaching CME?

"They're in no danger," says Hathaway. "The Space
Shuttle and the ISS are in low Earth orbit, well inside the magnetosphere.
The astronauts are protected by the magnetosphere and to some
extent by their own spacecraft. In fact, they're in a great position
to observe aurora if a geomagnetic storm breaks out."